An inspection of his patent reveals that his pump relies on the viscosity of the fluid to perform work- that is, the fluid is coupled to the impelling cylinder by shear forces alone. This means the fluid being pumped is being strongly sheared whenever the pump is running, and this inevitably produces significant frictional losses.
There are many other ways of pumping fluid in which the pumping forces are not generated by viscosity, and hence will not suffer the efficiency losses of Tesla's scheme.
The overall efficiency limit of this thermodynamic process can be accurately determined by calculating the carnot efficiency for the process, which limit holds regardless of the details of the process steps. This is given by:
Carnot efficiency =
(source temperature - sink temperature)/(source temperature * sink temperature)
where the source temperature is the temperature of the "hot side" of the cycle and the sink temperature is the temperature at which the cycle dumps its waste heat. All temperatures are in degrees absolute (Kelvin).